1. FIELD OF THE INVENTION
The present invention relates to an electrophotographic photosensitive element, a method of making such an element, electrophotographic apparatus including such an element and use of such apparatus. The invention particularly aims to provide an electrophotographic photosensitive element suitable for forming an excellent image even if used for printing in conditions of high humidity. Typically, the invention is applied to the electrophotographic element, e.g. drum, of an electrophotographic printer, such as a laser beam printer or a LED printer or a photocopier.
2. DESCRIPTION OF THE PRIOR ART
Electrophotographic photosensitive elements used in the prior art are exemplified by an inorganic photoconductive material such as Se, CdS or As.sub.2 Se.sub.3, or an organic photoconductive material as represented by a pigment of phthalocyanine.
These materials have excellent characteristics in respect of photosensitivity and charge acceptance but have the defect of short lifetime because their film hardness is too low to give a sufficient wear resistance. In contrast, an amorphous silicon photosensitive element is hard enough to achieve a long life time.
However, the amorphous silicon photosensitive element has the defect of inferior moisture resistance. It is, therefore, the current practice to form a surface protecting layer of a-SiC:H (amorphous SiC containing hydrogen) or a-SiN:H, which however is not sufficient to solve the problem that image blur occurs especially in high moisture conditions after repeated printing over a long time period.
This problem is thought to occur because the surface of the a-Si:H photosensitive element is oxidized by the corona discharge in the printing process of the electrophotography, which reduces its moisture resistance. In order to prevent this, it is necessary to use a chemically stable material as the surface protecting layer.
For example, there have been proposed a a-C:H:F:film as desclosed in Japanese Patent Application No. 62-22361, a thermoplastic resin film as disclosed in Japanese Laid-Open Patent Specification No. 55-70848, and a method using film a containing methyl silicon resin or a resin containing a fluorine compound as disclosed in Japanese Laid-Open Patent Specification No. 62-206559. These techniques are effective in improving the moisture resistance and the corona resistance but suffer from reduced wear resistance and cleanability.
Japanese Laid-Open Patent Specifications 64-56447, 64-56448, 64-56449 and 64-56450 describe a photosensitive element for copying machines and laser printers having a-Si:H photoconductive layer and a dense surface protective layer thereon. The outer face of the surface layer has considerable surface unevenness generated during its deposition, which is due at least partly to the unnevenness of the layer below. The surface indentations or cavities thus formed contain lubricant, e.g. hydrophobic lubricant, in the form of particles or in Japanese Laid-Open Patent Specification 64-56450 of melted and resolidified thermoplastic resin powder which may contain lubricant particles. The protective layer is made for example of a-SiC:H, Al.sub.2 O.sub.3 or high-resistivity polymer and may be 0.01-10 .mu.m thick.
The prior art thus far described cannot simultaneously satisfy the several requirements for the surface protecting layer of an electrophotographic photosensitive member, for example, moisture resistance, corona resistance, wear resistance and cleanability.